See Glen Shiel for the much smaller Loch Shiel in Lochalsh.

Loch Shiel (Scottish Gaelic: Loch Seile) is a freshwater loch situated 20 kilometres (12 mi) west of Fort William in the Highland council area of Scotland. At 28 kilometres (17+1⁄2 mi) long it is the 4th longest loch in Scotland, and is the longest to have retained a natural outflow without any regulation of its water level, being 120 m (393 ft) deep. Its nature changes considerably along its length, being deep and enclosed by mountains in the north east and shallow surrounded by bog and rough pasture in the south west, from which end the 4 km River Shiel drains to the sea in Loch Moidart near Castle Tioram.

The surrounding highlands are picturesque but relatively rarely climbed as none quite reaches the 3,000 ft (910 m) required for Munro status. A number of the hills are classified as Corbetts, including Beinn Resipol, Sgùrr Ghiubhsachain and Sgorr Craobh a' Chaorainn on the southern side of the loch; and Beinn Odhar Bheag on the northern side. The area is well wooded compared to the many Highland areas that have suffered from overgrazing. The view of the loch looking south from the Glenfinnan monument, showing wooded hillsides with bare summits rising steeply from a fjord-like loch, has become one of the most famous images of the Scottish Highlands. Several major films have used the loch for location shooting.

Loch Shiel is less than 10 metres above sea level, and was formed at the end of the last ice age when glacial deposits blocked what was formerly a sea loch.